The invention relates to an automatic downshift power transmission device and more specifically to an automatic downshift mechanism incorporating a wrap spring and planetary gear speed reduction assembly which automatically shifts between direct drive and reduced speed drive in accordance with the level of torque transmitted through the mechanism.
The majority of power transmission applications require the matching of speed and torque to the load for optimum performance. This results from the almost universal practice of utilizing a mechanical energy source such as an electric, pneumatic or hydraulic motor or internal combustion engine which delivers optimum power and torque over a relatively narrowly defined speed range. Thus, it is likewise nearly universal practice to couple the output of such a device to a mechanism such as a transmission which multiplies torque and reduces speed to match the output of the prime mover to the mechanical task assigned to it. The power train of an automobile comprising an internal combustion engine and an automatic or manual transmission represents a foremost example of this configuration.
The issue of speed and torque matching has also been addressed in connection with drive units for assembly tools. In this application, a fastener such as a nut, bolt, machine screw, self-threading screw or any threaded component will generally require low torque application as it is initially rotated prior to engaging the component or components which it will secure. When the fastener does engage such components, the torque required to rotate it will typically increase. The increased level of torque application is frequently monitored such that the torque ultimately applied to the fastener is appropriate to the application and consistent with the maximum torque the fastener may withstand without thread stripping or other damage, either to itself or the assembled components. A common conundrum of such devices relates to the need for relatively high speed, low torque power application during rundown of the fastener and higher, though carefully controlled, torque application near the end of the cycle to fully tighten the fastener.
I have expended great effort to develop torque control devices which not only limit torque application to a pre-selected level but also have reversible outputs or two speed outputs. For example, my U.S. Pat. No. 4,255,987, which issued Mar. 17, 1981, teaches a compact reversible speed reduction mechanism ideally suited for incorporation into assembly tools. The device includes a pair of juxtaposed epicyclic gear trains mounted in a common carrier which independently engage a pair of ring gears to provide bi-directional output rotation.
A device providing a similar function is disclosed in my U.S. Pat. No. 4,364,286 which issued Dec. 21, 1982. Here, a pair of epicyclic gear trains mounted in a common carrier engage respective ring gears which are locked or released by a respective pair of wrap springs disposed about the cages.
My United States reissue patent No. Re. 33,514 teaches a torque limiting assembly having a pair of coaxially disposed wrap springs; the first spring providing an adjustable torque transmission limit and the second providing a mechanical signal that the torque limit has been reached.
Others have addressed this area of technology as well. For example, U.S. Pat. No. 4,328,871 to Gluskin granted May 11, 1982 teaches an assembly which provides both a high speed/low torque mode for running down a threaded fastener and a low speed/high torque mode for tightening the fastener. The device utilizes two wrap springs: an inner wrap spring which locks and unlocks a planetary gear assembly and an outer wrap spring which is controlled by an actuator to control the gripping or release of the first wrap spring.
U.S. Pat. Nos. 4,869,139 and 4,991,473 of Gotman, the former which issued Sep. 26, 1989 and the latter which issued Feb. 12, 1991, both disclose an automatic speed and torque switching or shifting device. In this device, two different drive trains having distinct torque and speed outputs are driven in parallel by a common motor. Initially, the high speed, low torque output is utilized to drive the fastener. When the level of torque applied to the fastener increases, the coupling to the high speed drive train is disabled and the low speed, high torque output is utilized.
The foregoing discussion suggests not only the desirability of such dual speed/dual torque drive mechanisms which automatically select the appropriate drive speed and torque level output but also reveals the fact that such devices tend to be complicated and may require torque level sensors and auxiliary components to achieve their stated goals. Accordingly, it is apparent that improvements in the art of dual speed and torque delivering mechanisms which automatically shift between such outputs are desirable.